The Height of the Bomb

by Alex Wellerstein, published August 8th, 2012

The United States justly gets the lion’s share of credit for making and using the atomic bomb. The Manhattan Project was an international effort, we all know, but the U.S. bore the greatest amount of the labor, the cost, and the ultimately responsibility. When people get mad about the bomb, they get mad at the United States. It makes sense.

The importance of the United Kingdom, in some narratives of the bomb, was mostly in pushing the idea, sending a handful of scientists to help, and signing off on the use of the bomb. (Let’s not even get into Canada, who, according to this narrative, just provided some uranium and a lab that wasn’t allowed to communicate with the rest of the Manhattan Project.)

Several members of the British Mission to Los Alamos: William Penney, Otto Frisch, Rudolf Peierls, and John Cockroft. Photo via Los Alamos.

That handful of scientists, though, included some fairly important people. James Tuck did crucial work relating to the development of the explosive lens in the Trinity and Nagasaki bombs. Klaus Fuchs, aside from being the most significant Soviet spy on the project, helped tremendously with the design of the implosion bomb and the neutron initiators. And so on.

William Penney (later Baron Penney), who later ran the British atomic bomb project, did a whole variety of interesting things. He was crucial in developing means of assessing the effects of the bomb, was the only member of the British delegation to witness the bombing of Japan (he was on an observation plane on the Nagasaki mission), and was one of the first Allied scientists who went into Hiroshima and Nagasaki after the war.

One of the other topics he worked on was the question of what height the bombs should be detonated at for maximum effect.

Illustration of one of the effects of a bomb detonated in the air: a reflection of the blast wave off of the ground, which produces a powerful shock wave known as a Mach stem. From Glasstone and Dolan’s The Effects of Nuclear Weapons(1957 edn.).

This was a non-trivial issue: prior to nuclear weapons, it was pretty common for a bomb to detonate just when it actually physically made contact with its target. With atomic bombs, though, that isn’t ideal. Set one off too high, and much of the blast is going off into outer space and not the target. Too low, and a lot of it is going off straight into the ground. There is no truly “right” height, in the sense that any height will produce different effects that may or may not be desirable, but if the goal is the destruction of civilian buildings, then there are heights that work much better for that than others.

All of which Penney wrote up in a charming December 1944 report on “The Height of Burst of the Gadget.”1

Click for the PDF.

It’s a chilling — and fascinating — document for a few reasons. One is that it goes against any myth that the height of the bomb was set to minimize fallout exposure. This is discussed in great length by Sean Malloy in a recent article in Diplomatic History, which I will be posting a review of fairly soon.2The entire reasoning for the height of the bomb detonation was about blast and fire effects. Radiation didn’t come into it. Later, when it was questioned as to whether there were fallout issues, the Manhattan Project principals said that they thought the height of the blast would avoid a lot of fallout — which is somewhat true — but this isn’t what caused them to set the height of the blast.

Another interesting bit is that Penney actually wants you to set the bomb off very low — 500 feet or so above the ground — rather than very high. The reason, though, is that Penney is assuming a much smaller bomb than actually was built: his calculation is based on the notion that the bomb will only be about 1 kiloton in yield, whereas previous guesses had been that it would be 10 kilotons or so in yield.

William Penney in the 1950s, looking like a character out of a Le Carré novel

Penney’s document is remarkable in its frankness about the point of the bomb: to destroy civilian houses. He also makes a big distinction between the types of targets available:

There is a significant difference in the blast resisting characteristics of German towns and of Japanese towns. While in Germany a broad distinction may be made between industrial buildings and the remainder, it is true that there is no pronounced contrast in the strength of one building or another, with the exception of the multi-floored reinforced concrete structures which are relatively few in number. In Tokyo or Yokahama, some areas consist entirely of extremely strong steel framed and concrete structures built to resist earthquakes, whereas other much larger areas contain only comparatively flimsy wooden houses with tiled roofs. … Therefore, it may be said that if the gadget is to be used in area attack against a German town, only one height of fusing is required for attack anywhere. Such is not the case for area attack on Tokyo. If the accuracy of delivery can be guaranteed within 500 yards, then the bursting height for attack on wooden houses can be set twice as high as for attack on the business and shopping areas. If this accuracy cannot be guaranteed, then some suitable compromise must be worked out from tactical and statistical arguments.

In other words, all German buildings would be more or less affected in the same way in an atomic bomb blast. Japanese buildings, however, vary between very strong industrial buildings and very flimsy civilian buildings. If the goal is to target civilians, you have much more leeway in the blast height.

Two very interesting things here: One, an atomic attack on Germany is still being explicitly discussed. Two, the “area attack” being discussed is one that contemplates the direct and purposeful targeting of civilians — perhaps even at a detriment to targeting industrial facilities.

Penney then moves on to consider what kind of damage you’re trying to maximize: “complete destruction” versus “severe but not unrepairable damage.” Penney notes that “opinion in England” has gone from favoring “severe” damage to favoring “complete destruction.” Why? Because it turns out that “severe” damage is pretty quick to repair, as the Londoners discovered after the V-1 attacks (“the robot attacks,” as Penney calls them), and because “complete destruction” “implies many casualties, and this in turn has a very serious effect on the efficiency of fire fighting.”

Dwell on that for a moment. The more people you kill, the less people who can put out the fires. Lest you think this is perhaps just a moment of remarkable cruelty, Penney actually elaborates on this theme at length later in the report. He makes a distinction here between the “Fire Force” and “fire guards,” which, if I understand it correctly, is the distinction between professional and amateur fire prevention forces:

The explosion of a gadget in either Germany or Japan, causing large amount of [“complete destruction” class] damage will almost certainly result in fires. While the general impression among the Fire Force in England is that the Japanese are likely to prove the most efficient fire guards in the world (because their ordinary lives gives them continual experience), the large number of casualties associated with [this class of] damage may well lead to such confusion in Japan that the critical incubation period of the fires [to become a firestorm] is passed unobserved. Thereafter the fire guards are useless, and only the Fire Force counts. The possibility of eliminating a large fraction of the Fire Force of a Japanese town by getting the fireman into the radioactive contaminated area to fight fires is attractive and realistic. The success of a follow-up attack may be greatly increased in this way. … If a gadget can be followed or accompanied by small [incendiary devices] the probability of a devastating fire, spreading well beyond the limits of the blast damage, will be greatly increased.

Gristly thoughts, no? This is also the only area of the report where radioactivity is mentioned: as a way to kill firefighters, so you can encourage a massive conflagration to develop.

Fire damage in Hiroshima shown in red

In reality, a massive firestorm did erupt in Hiroshima and Nagasaki, and was responsible for the majority of the deaths in each. Making the bombs go off at just the right height for maximizing this sort of damage was a high priority — it even required the development of new kinds of bomb fuses just for this purpose.3

The bomb that went off above Hiroshima, 67 years ago this week, was set off at 600 meters above the ground. The Nagasaki bomb was detonated at 500 meters. Both of those heights were chosen to maximize damage — especially for the “flimsy” wooden houses of Japanese civilians.

It’s easy to demonize the people who made these kind of decisions — especially when they describe the pre-irradiation of firefighters as “attractive.” But this is the logic of total war, when you’ve given up on the idea of a morality of war fighting and decide that the ends — and not the means — are all that matter.

Whether one thinks the bombing of Hiroshima and Nagasaki were justified or not, it must be remembered that along with whatever else they were they were massacres of civilians. It was not an incidental or accidental side-effect: it was what they were planned to be. When the scientists at Los Alamos made plans for how to use the atomic bomb, they optimized them for the burning of civilians. This should not be forgotten or glossed over, even if one thinks it was still the right thing to do.

Sean L. Malloy, “‘A Very Pleasant Way to Die’: Radiation Effects and the Decision to Use the Atomic Bomb against Japan,” Diplomatic History 36, no. 3 (June 2012), 515-545. [↩]

The Little Boy and Fat Man bombs had two sets of altitude-detecting fuses. One, known as the “Archie,” was based on a small directional antennae that had been originally developed so that airplanes could tell if another airplane was behind them. The other was a barometric switch that relied on the changes in atmospheric pressure at different altitudes. They also had a backup timer system that would go off if the other two failed. The technical specifications of both of these fuses are long since declassified, and even patented! See the “Folded Dipole” and “Pressure Sensitive Switch” patents on my page of atomic patents. [↩]

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12 Responses to “The Height of the Bomb”

I wonder if Penney was inspired to consider fire damage by the RAF’s extensive use of blockbuster bombs on German cities. High-explosive blockbuster (or cookies, to use the RAF’s grimly humorous term) would destroy the roofs and windows of brick or stone house, exposing the wooden rafters and floors. Followup raids with incendiary bombs would then be more effective.

What’s interesting about the report is that it seems like he was less using data about bombing on Germany (though he did have some of that) than data about German bombing on England. His idea about mixing the H.E. and incendiary effects came from the Blitz, and his understanding about the value of killing off the firefighters came from experience when the British Fire Force had gotten extremely low due to deaths.

An interesting observation is in Section VII is Penney doesn’t make explicit that the “gadget” might generate a firestorm (or “fire tornado” as Bomber Harris called them in 1947) but he does mention large areas a “C” level damage and implications for a follow-up “match bomb” incendiary raid.

I find it interesting as this report was written in December 1944. The bombing of Hamburg on 27 July 1943 created the first firestorm of WW2 though the effects of fire driven winds from aerial bombing were observed during the Blitz in 1940. Other firestorms followed in 1944 and most in 1945 in Germany and Japan.

I’ve always presumed the idea of the firestorm was fully understood before the bombing of Hiroshima and Nagasaki but perhaps not?

How did the idea of a fire-storm evolve in WW2 in planning for both conventional and nuclear bombing?

Was a fire-storm explicitly accounted for in the planning of the final burst height of the A-bombs?

In Section VII on “radiation contamination” does Penny mean the effects of fallout from the bomb or the effects of prompt radiation including neutrons irradiating material on the ground and so making that material alpha/beta/gamma radioactive? Or was he just thinking of prompt gamma and prompt neutrons?

Kevin,
The firestorm concept may not have been fully fleshed out publicly during WWII, but Penney was well aware of the basics mechanics. The Allies had several choices of incendiary weapons to use, but they all required massed aircraft formations to bring about a high enough concentration of fire to get the storm going. To a certain extent, firestorms also depended on weather conditions and, of course, the nature of the built-up areas being attacked. European cities required greater effort to burn, while Japanese cities were easier due to the light and more easily set afire construction methods and materials.

Penney’s reference to radiation was most likely a reference to gamma and prompt radiation. The risks of alpha and beta radiation was still poorly understood. Burns from beta radiation were noticed as a problem among those handling “hot” samples for test diagnostics and intel R&D during the 1948 SANDSTONE test series. A full appreciation of the dangers of alpha wouldn’t be recognized until near the end of the 1950s as the risks posed by fallout were better understood.

What’s really interesting was how the problem of nuclear firestorm was largely suppressed during the Cold War. Lynn Eden’s 2004 book, Whole World on Fire: Organizations, Knowledge and Nuclear Weapons Devastation covers this well, although I’ve only read the excerpts published IIRC in the Bulletin of the Atomic Scientists.

This is particularly interesting in my research, as the Air Force tended to see nuclear weapons as just more powerful forms of conventional weapons, which is one reason why they tended to ignore the problem of fallout. This casting of nuclear weapons as simply hyper-efficient conventional ones was a way to suppress the moral issues raised by strategic attacks on civilian targets, which became widely acceptable in the course of WWII. Apparently, the US military was nearly as reticent to address the use of firestorms against cities as they were to address the risks of fallout, indicating that even they saw such attacks as close to the moral line shading into war crimes that they preferred to avoid an open and thorough discussion of the issue.

It’s unclear to me whether Penney meant prompt radiation or fallout. I have a review of an excellent article on this question (Malloy’s, cited here) coming out extremely soon. Some physicist were well-aware of fallout, some were not. The question of fallout never really filtered up the chain of command, though. I’m unsure whether Penney himself was thinking of it or not. The fact that he uses the term “contaminated” makes me think he is talking about something more long-term than prompt radiation, though.

Alex,
You may be right that Penney and some of the other scientists did understand more about fallout than they were able to persuade their superiors of, but this would’ve been simply theoretical when Penney was musing about the height of the bomb before it was first used. I’m less well versed in the British side of things, but Hacker documented well how the AEC focused on gamma as the primary threat from testing well into the 1950s. My guess is that Penney held much the same view, although that’s just speculation on my part.

I think the “contamination” idea may be related to some of the concerns they still held about the German atomic research program. While the nuclear intelligence work of ALSOS had largely ruled out the possibility of a German fission bomb by the beginning of 1945, there were still concerns that the Germans might have been able to develop mixes of deadly isotopes in lieu of actual fission. Like some other ways of war, once the Germans adopted it, it became justifiable for the British to to go there, too, so Penney may have been keeping this option open for this reason.

I’m basing my assessment of Penney on his later role in shutting down some of the fallout research that accompanied British nuclear testing in Australia. I’m referring here to the Hedley Marston affair, where Penney played a role in retrieving the equipment that allowed Marston to closely inquire into the role of I-131, one of the key isotopes used for nuclear intelligence purposes.

Penney obviously knew more about fallout by the mid-50s, but his reaction to independent scientific research into it was to suppress it. I believe that Penney’s motivations at the time were primarily related to keeping secret the techniques that the British and Americans used for nuclear intelligence, but he was also leery of stirring up the Australian public, which eventually did indicate they would prefer the British do their testing elsewhere.

Well, let me just assure you, you would find the Malloy article interesting. I’ll post a link to my review of it when it’s up and running. It’s remarkable what was and wasn’t known about fallout and fission products in 1944-1945, and who knew it and who didn’t. You’re right that the question of radiological warfare directly plays into this sort of assessment. Those who did discuss fallout in 1945ish were mostly talking about short-lived fission products, though — I don’t think they were thinking in terms of radioactive iodine or strontium, yet.

Alex,
Had a chance to quickly read through Malloy’s article. Some very useful stuff there.

My diss does include some Manhattan Project stuff, but Malloy provides a couple of excellent cites (which are actually quite handy) and offers intriguing suggestions that may cause me to recast a couple of things I did manage to include. Given the Ziegler and Jacobson covered nuclear intelligence up through 1949 and JOE-1, I pretty much moved on from there with my diss, which concentrates on 1949 to 1964.

I do have a couple of observations. Groves has a lot on his plate and tended to defer to Oppenheimer about what was important scientifically, thus I can easily see “fallout before it had a name” being something that was a matter more of neglect than intent given everything else going. If Oppie wasn’t worried about it, why should Gives be?

Then there was the issue of his fears over the darn bomb just not working and how that would put him on the spot after spending all that money in secret. Having the bomb turn out to be “poisonous” wouldn’t be quite the disaster that failure to explode would’ve been, although that is a bit more self-serving than simple neglect as an explanation if that had Groves worried. Better to ask forgiveness, than risk asking permission, if it did trun out to be a problem? Works for teenagers and politicians, why not generals?

Then there is the complexity of such an enormous organization operating in wartime. Given the press of other priorities, it’s not unreasonable to say this just fell through the cracks, although it’s obvious that a few folks do seem to be on the right track, although empirical evidence for their theory that radiation would be a threat was still not present.

What’s key in my diss argument is something similar, based on what Z&J documented for the R&D involved in chasing fallout for intelligence purposes. It was really hard at first, so this tended to support the idea in the minds of the military that fallout couldn’t be much of an issue if it was so hard to track down.

One thing that is pretty clear is that Oppenheimer’s dismissive view on fallout as inconsequential changed. At the very least, he evidenced a lot of enthusiasm in helping with R&D after SANDSTONE experiments finally proved that collecting aerial samples at a distance could work. The problem there was that the Air Force grew pretty hostile about having him around shortly after JOE-1. But Oppie did seem to finally embrace the idea that fallout was a threat with the super, simply because of the quantities involved. So he did respond to empirical evidence in terms of his changing views on fallout.

The problem then was that the military assumed that he was simply making an issue of fallout as one of the many reasons they thought he was finding to oppose pursuing the super. It took CASTLE BRAVO to eventually make them reluctant believers, too, and the resistance to changing what many in the military believed about fallout, that it was inconsequential, continued for some time as influential in the way the military approached the subject and still persists to a certain degree.

Thus, it is amazing to see several very familiar characters involved at this early date in a fallout cover up. One of the S. Warrens, I’m pretty sure it was Stafford (I can’t keep them straight without my notes in front of me) was the person who certified that using the Nevada Test Site would be “safe,” for instance. I’m just not sure there was any collective or even completely malicious intent to bury this info this early, but it does demonstrate the natural inclination of those who had it that it was a very sensitive topic and kept from view, if at all possible. Certainly this was an approach to fallout that prevailed for the next couple of decades, an example of what I call infrapolicy, which is a lot like policy, but functions in its absence or at variance with official statements. The Pentagon was more than happy to try to keep this buried for multiple reasons, but fallout has the power of tending to escape and seep into everything…and well, there’s a lot more. Looking forward to your review.

[…] the bombs more deadly. Whether it was in racing towards a megaton age (Teller’s approach), or calculating the best way to kill Japanese firefighters (Penney’s approach), or — the subject of a future post — a proposal for generating […]

[…] contrary, the targeting height of the bomb was not chosen in order to minimize radiation effects. It was chosen to maximize blast and thermal effects. The argument that its height was chosen to minimize radiation effects is an […]

[…] So the overall point here is that blast reflection can dramatically increase the blast pressure of the bomb at the point where it occurs. But the location at this point varies depending on the height of the bomb detonation — so you can use the choice of bomb detonation altitude to maximize certain pressures in particular. So this is what the Target Committee was talking about in May 1945: they wanted to maximize the radius of the 5 psi overpressure range, and they recognized that this involved finding the correct detonation height and knowing the correct yield of the bomb. They knew about the reflection property and in fact referred to the Mach stem explicitly in their discussion. Why 5 psi? Because that is the overpressure used to destroy “soft” targets like the relatively flimsy houses used by Japanese civilians, which they had already realized would be much easier to destroy than German-style houses. […]